We have proposed a hypothesis which states that embryonic cells of echinoderms possess specificity of adhesion according to species and cell type and that this adhesion is mediated by the specific formation of microvilli and extracellular material. We plan to complete the isolation and characterization of proteins present in the extracellular material, which we have previously demonstrated are involved in both specific and non-specific cell adhesion and reaggregation. We will also isolate the cell surface receptor molecules for these proteins and study their ontogeny by making use of hybrid embryos. The synthesis and release of these molecules by embryonic cell types will also be explored. Finally, we propose to study further the effect of cell dissociation and reaggregation on the capacity for, and rate of development of embryonic cells.